500-year half-life? Yep.
>I'd be a little sceptical about the claimed half-life.
I'd rather look at the evidence myself - and an internet search isn't that bothersome, is it? A half life of 500 years has been observed in the DNA from bones of Moa, extinct birds, dating from between 600 to 8,000 years, preserved in similar conditions.
Were this DNA archival process ever to be used, there is no reason why the archive couldn't be based somewhere cold - much like the Svalbard Global Seed Vault. (https://en.wikipedia.org/wiki/Svalbard_Global_Seed_Vault#Construction )
Then of course error correction methods and redundancy can be built into any DNA-archival process.
You mutations you mention are those seen in living cells, and usually occur during the copying stage (and yeah, our cells have several error-correcting mechanisms) - but this is very different to these inert strands of DNA that have been removed from the molecular machinery.